Relatively pure metal oxides are produced by thermal decomposition of precursors and deposition of the resulting oxides. The precursor may take the form of a vapor, or may be carried by a vapor. It may be decomposed by either flame hydrolysis or pyrolysis.
One such process is production of fused silica by hydrolysis or pyrolysis of silicon tetrachloride. Early patents disclosing such processes for producing silica are U.S. Pat. Nos. 2,239,551 (Nordberg) and 2,272,342 (Hyde). A commercial application of flame hydrolysis involves forming and depositing particles of fused silica to form large bodies (boules). Such boules may be used individually, or may be finished and integrated together into large optical bodies, such as telescope mirrors. In this procedure, SiCl.sub.4 is hydrolyzed, and the hydrolyzed vapor is passed into a flame to form molten particles of fused silica. The particles are continuously deposited on a bait, or in a crucible, known as a cup, to form a boule.
A serious drawback in this process has been the need to dispose of the HCl by-product in an environmentally safe manner. Accordingly, it has been proposed, in U.S. Pat. No. 5,043,002 (Dobbins et al.), to employ a halide-free, silicon-containing compound as a substitute for SiCl.sub.4. In particular, the patent proposes using a polymethylsiloxane, such as octamethylcyclotetrasiloxane, to provide the vaporous reactant for the hydrolysis or pyrolysis process.
In order to introduce a substitute precursor, it is, of course, critically necessary to avoid any significant change in the properties of the fused silica product. Unfortunately, the substitution proposed by the Dobbins et al. patent did lead to significant property changes. One such change was a reduction in UV transmission properties. Another was development of fluorescence in the glass that increased when the glass was exposed to short wavelength radiation.
Studies revealed that a factor in the transmission loss was sodium ion content in the glass. U.S. Pat. Nos. 5,332,702 and 5,395,413 (Sempolinski et al.) describe remedial measures taken to reduce the sodium ion content. Essentially, these measures constituted providing a purer zircon refractory for use in constructing a furnace in which the fused silica was deposited to form a boule. In particular, it was found necessary to use dispersants, binders and water relatively free of sodium ions in producing zircon refractory components for the furnace.
An improved product was obtained by adopting the practices prescribed in the Sempolinski et al. patents. However, use of the fused silica in certain applications made it apparent that further improvements were necessary to meet the critical requirements of these applications. One such application is lenses designed for transmission of very short UV wavelength radiation from an excimer type laser. This laser emits radiation at about 193 nm and 248 nm wavelengths.
It was found that lenses produced from available fused silica did not provide acceptable transmission of the short wavelength radiation and exhibited an undesirable fluorescence. Both of these conditions tend to become worse with service time. The loss of transmission, or darkening of the glass, is commonly referred to as UV absorption damage.
It is a primary purpose of the present invention to provide a fused silica material that alleviates these problems. Another purpose is to provide an improved fused silica glass for lenses used with lasers, especially for microlithographic work. A further purpose is to provide an improved furnace for collection of the fused silica in the form of a boule. A still further purpose is to provide a method of achieving the improved collecting furnace and glass produced therein.